Electrodes implanted in the body for electrical cardioversion or pacing of the heart are well known. More specifically, electrodes implanted in or about the heart have been used to reverse (i.e., defibrillate or cardiovert) certain life threatening arrhythmias, or to stimulate contraction (pacing) of the heart, where electrical energy is applied to the heart via the electrodes to return the heart to normal rhythm. Electrodes have also been used to sense near the sinus node in the atrium of the heart and to deliver pacing pulses to the atrium. An electrode positioned in any chamber of the heart senses the electrical signals that trigger the heartbeat. Electrodes detect abnormally slow (bradycardia) or abnormally fast (tachycardia) heartbeats. In response to the sensed bradycardia or tachycardia condition, a pulse generator produces pacing or defibrillation pulses to correct the condition. The same electrode used to sense the condition is also used in the process of delivering a corrective pulse or signal from the pulse generator of the pacemaker.
There are four main types of pulses or signals which are delivered by a pulse generator. Two of the signals or pulses are for pacing the heart. First of all, there is a pulse for pacing the heart when it is beating too slowly. The pulses trigger the heart beat. These pulses are delivered at a rate to increase the abnormally low heart rate to a normal or desired level. The second type of pacing is used on a heart that is beating too fast. This type of pacing is called antitachycardia pacing. In this type of pacing, the pacing pulses are delivered initially at a rate much faster or slower than the abnormally beating heart until the heart rate can be returned to a normal or desired level. The third and fourth types of pulses are delivered through large surface area electrodes used when the heart is beating too fast or is fibrillating, respectively. The third type is called cardioversion. This is delivery of a relatively low energy shock, typically in the range of 0.5 to 5 joules, to the heart. The fourth type of pulse or signal is a defibrillation signal which is the delivery of a high energy shock, typically greater than 25 joules, to the heart.
Sick sinus syndrome and symptomatic AV block constitute the major reasons for insertion of cardiac pacemakers today. Cardiac pacing may be performed by the transvenous method or by electrodes implanted directly onto the epicardium. Most commonly, permanent transvenous pacing is performed using one or more leads with electrodes positioned within one or more chambers of the heart. The distal end of a lead, sometimes referred to as a catheter, may be positioned in the right ventricle or in the right atrium through a subclavian vein. The lead terminal pins are attached to a pulse generator which is implanted subcutaneously.
Some patients require a pacing system to detect and correct an abnormal heartbeat in both the atrium and ventricle which may have independent rhythms, as well as a defibrillation system to detect and correct an abnormally fast heart rate (tachycardia condition). In the past, a common practice for a patient having to pace both of these chambers would be to provide two different leads attached to the heart. One would be implanted for delivering pacing/sensing/defibrillating to the ventricle and one to the atrium to both pace and sense.
Having two separate leads implanted within the heart is undesirable for many reasons. Among the many reasons are that the implantation procedure for implanting two leads is more complex and also takes a longer time when compared to the complexity and time needed to implant a single lead. In addition, two leads may mechanically interact with one another after implantation which can result in dislodgment of one or both of the leads. In vivo mechanical interaction of the leads may also cause abrasion of the insulative layer along the lead which can result in an electrical failure of one or both of the leads. Another problem is that as more leads are implanted in the heart, the ability to add other leads is reduced. If the patient's condition changes over time the ability to add leads is restricted. Two separate leads also increase the risk of infection and may result in additional health care costs associated with re-implantation and follow-up.
Because of these problems, catheters having electrodes for both pacing and sensing in both chambers of the heart on a single lead body have been used. These leads, known as single pass lead designs, have drawbacks since the single pass lead designs utilize "floating" electrodes or electrodes which are not attached to the endocardial wall of the heart. The catheter having the electrodes which forms the lead body is essentially straight. The electrode or electrodes may float or move slightly at a distance from the endocardial wall within the heart.
The portion of the lead positioned within the atrium of current single-pass endocardial leads has one or more electrodes which are incorporated into the lead body as an electrically conductive cylindrical or semicylindrical ring structure. In other words, the lead body is basically cylindrical and the one or more electrodes positioned within the atrium of the heart are cylindrical metal structures incorporated into the cylindrical lead body. The ring electrode structures do not allow for tissue ingrowth into the electrode to enhance electrode stabilization within the atrium. Since the location of the electrodes is not fixed against the atrial wall, the performance of these leads is more variable. In other words, variations with respect to electrical contact with the wall of the atrium results in suboptimal electrical sensing capability and pacing delivery capability. Typically, the pacing characteristics of a floating electrode are less desirable than the pacing characteristics associated with an electrode fixed to the endocardial wall of the heart. The performance of a lead using a floating electrode is poorer than a lead having electrodes which contact or are nearer the walls of the heart.
Another problem associated with the current straight single pass leads, is that these electrodes may be unable or less able to sense an arrhythmic condition. In addition, the applied voltage or current needed for pacing may be ineffective. Additional energy may have to be used to pace the heart thereby depleting energy from the battery of the pulse generator of the pacing system.
There is a real need for a single-pass transvenous pacing or defibrillation lead. A single-pass lead equipped with such an electrode arrangement would allow for better sensing capability and better pacing therapy to the heart. In addition, there is a need for a single-pass lead having an electrode for positioning within the atrium that allows for tissue ingrowth. Such an electrode would further enhance lead stabilization within the heart. There is also a need for a single-pass endocardial lead which has an electrode for placing within the right atrium of the heart that accommodates eluting anti-inflammatory drugs. There is still a further need for a single pass endocardial lead that is easier for a surgeon to implant.